The process for making monoacetals of hydroquinone is disclosed in co-pending patent application U.S. Ser. No. 08/206,573, filed Mar. 4, 1994, now abandoned; incorporated herein by reference. Said process involves reacting equimolar amounts of hydroquinone with enol ethers in the presence of an acid catalyst: ##STR1## (i) each R is, independently, selected from the group consisting of hydrogen; C.sub.1 -C.sub.10 alkyl; benzyl, aryl and substituted benzyl or aryl.
(ii) each R' is, independently, selected from the group consisting of C.sub.1 -C.sub.10 alkyl; benzyl, aryl and substituted benzyl or aryl. PA1 (iii) n is an integer from 0 to 3. PA1 a) reacting monoethers of hydroquinone with an enol ether in the presence of an acid catalyst to yield the protected monoacetal of hydroquinone as an intermediate; and PA1 b) reacting said intermediate with a hydrogen transfer source in the presence of a metal catalyst such that protecting group is selectively cleaved to give desired hydroquinone monoacetal.
However, said reaction has several limitations. For equimolar concentrations of hydroquinone and enol ethers said reaction also results in formation of hydroquinone bis-acetals which are poor skin lightening compounds. Removal of the non-efficacious bis-acetal compounds, along with unreacted hydroquinone, necessitates expensive large scale chromatographic purification such that isolated yield of said monoacetal is poor, typically 30-40%. Attempts at improving overall yield and cost of process via conversion/recycling of bis-acetal to desired monoacetal is problematic due to similar reactivity of the two compounds, while changing the stoichiometry of the reaction to minimize bis-acetal formation increases the amount of hydroquinone which ultimately needs to be chromatographically removed.